Boron synergies in lithium film performance

Abstract

Fusion experiments use lithium and boron getters to capture impurities and control the density of fuel species in the plasma. In the TJ-II Stellarator, the addition of a boron-carbon substrate was found to extend the lifetime of the very reactive lithium wall conditioning. However, the synergies between the lithium layer and the underlying boron-carbon, or the effect of a glow discharge cleaning, on oxygen and deuterium gettering are not fully understood. Laboratory experiments were therefore initiated to gain deeper insight on the getter properties of B(C)/Li walls. The coatings were exposed to oxygen gas and plasmas while the capture and release of O was followed by mass spectrometry. Without the boron substrate, the maximum oxygen uptake is half the number of lithium atoms in the film for both oxygen gas and oxygen plasma. Conversely, the behaviour of B(C)/Li walls depends on the nature of the oxidation process: molecular oxygen (O:Li ≈ 0.15) or plasma (O:Li ≈ 0.50). The oxygen plasma gettering is thus preserved against molecular oxidation, e.g. an overnight exposure to the residual gas. While oxygen is known to promote hydrogen retention in lithium, the oxygen content of a B(C)/Li coating shows little effect on deuterium retention. Unlike simple lithium, the oxygen and deuterium gettering of B(C)/Li recovers after a helium glow discharge treatment. These advantageous features clearly point to a change in chemistry, with complex interactions between film constituents and the oxygen-rich environment of magnetic fusion devices.